A platelet-mediated system for shuttling blood-borne bacteria to CD8α+ dendritic cells depends on glycoprotein GPIb and complement C3.

The acquisition of pathogen-derived antigen by dendritic cells (DCs) is a key event in the generation of cytotoxic CD8(+) T cell responses. In mice, the intracellular bacterium Listeria monocytogenes is directed from the blood to splenic CD8α(+) DCs. We report that L. monocytogenes rapidly associated with platelets in the bloodstream in a manner dependent on GPIb and complement C3. Platelet association targeted a small but immunologically important portion of L. monocytogenes to splenic CD8α(+) DCs, diverting bacteria from swift clearance by other, less immunogenic phagocytes. Thus, an effective balance is established between maintaining sterility of the circulation and induction of antibacterial immunity by DCs. Other gram-positive bacteria also were rapidly tagged by platelets, revealing a broadly active shuttling mechanism for systemic bacteria.

What if protective immunity is antigen-driven and not due to so-called "memory" B and T cells?

Vaccines or early childhood exposure to infection mediate immunity, that is, improved resistance against disease and death caused by a second infection with the same agent. This has been explained by and equaled to immunological memory, that is, an "altered immune system behavior" that is maintained in a presumably antigen-independent fashion. This review summarizes epidemiological and experimental data, that largely falsify this idea and that show that periodic re-exposure to antigen either, artificially as vaccines or naturally as low-level persisting antigens or infections, or immune complexes on follicular dendritic cells or endemic re-exposure is necessary for protection. Both, the huge success of vaccines in controlling childhood infections, the reduction in clinical disease and the chance of endemically re-exposure, have gradually reduced periodical re-exposure to infections and thereby endangered protective herd immunity. In parallel, vaccine deniers have created susceptibility islands even in an otherwise well vaccinated population, thereby creating a very new situation when compared to the later parts of the 20th century. If protective Immunity is-as emphasized here-antigen driven, then increasingly frequent revaccinations will be necessary (even more so with too much attenuated vaccines) to maintain both herd immunity and individual resistance to acute infections. Of course, this rule also applies to tumor vaccines.

Antiviral antibody responses: the two extremes of a wide spectrum.

Viruses elicit a diverse spectrum of antiviral antibody responses. In this review, we discuss two widely used experimental model systems for viral infections - non-cytopathic lymphocytic choriomeningitis virus (LCMV) and acutely cytopathic vesicular stomatitis virus (VSV) - to analyse two fundamentally different types of antiviral antibody response. The basic principles found in these model infections are discussed in the context of other viral infections, and with regard to protective neutralizing versus non-protective enzyme-linked immunosorbent assay (ELISA)-detected antibody responses. Issues of antibody specificity, affinity and avidity, maturation and escape are discussed in the context of co-evolution of the host and viruses.

Extralymphatic virus sanctuaries as a consequence of potent T-cell activation.

T helper cells can support the functions of CD8(+) T cells against persistently infecting viruses such as murine lymphocytic choriomeningitis virus (LCMV), cytomegalovirus, hepatitis C virus and HIV. These viruses often resist complete elimination and remain detectable at sanctuary sites, such as the kidneys and other extralymphatic organs. The mechanisms underlying this persistence are not well understood. Here we show that mice with potent virus-specific T-cell responses have reduced levels and delayed formation of neutralizing antibodies, and these mice fail to clear LCMV from extralymphatic epithelia. Transfer of virus-specific B cells but not virus-specific T cells augmented virus clearance from persistent sites. Virus elimination from the kidneys was associated with the formation of IgG deposits in the interstitial space, presumably from kidney-infiltrating B cells. CD8(+) T cells in the kidneys of mice that did not clear virus from this site were activated but showed evidence of exhaustion. Thus, we conclude that in this model of infection, site-specific virus persistence develops as a consequence of potent immune activation coupled with reductions in virus-specific neutralizing antibodies. Our results suggest that sanctuary-site formation depends both on organ anatomy and on the induction of different adaptive immune effector mechanisms. Boosting T-cell responses alone may not reduce virus persistence.

Immunological memory ≠ protective immunity.

So-called 'immunological memory' is, in my view, a typical example where a field of enquiry, i.e. to understand long-term protection to survive reexposure to infection, has been overtaken by 'l'art pour l'art' of 'basic immunology'. The aim of this critical review is to point out some key differences between academic text book-defined immunological memory and protective immunity as viewed from a co-evolutionary point of view, both from the host and the infectious agents. A key conclusion is that 'immunological memory' of course exists, but only in particular experimental laboratory models measuring 'quicker and better' responses after an earlier immunization. These often do correlate with, but are not the key mechanisms of, protection. Protection depends on pre-existing neutralizing antibodies or pre-activated T cells at the time of infection-as documented by the importance of maternal antibodies around birth for survival of the offspring. Importantly, both high levels of antibodies and of activated T cells are antigen driven. This conclusion has serious implications for our thinking about vaccines and maintaining a level of protection in the population to deal with old and new infectious diseases.

Nonneutralizing antibodies binding to the surface glycoprotein of lymphocytic choriomeningitis virus reduce early virus spread.

The biological relevance of nonneutralizing antibodies elicited early after infection with noncytopathic persistence-prone viruses is unclear. We demonstrate that cytotoxic T lymphocyte-deficient TgH(KL25) mice, which are transgenic for the heavy chain of the lymphocytic choriomeningitis virus (LCMV)-neutralizing monoclonal antibody KL25, mount a focused neutralizing antibody response following LCMV infection, and that this results in the emergence of neutralization escape virus variants. Further investigation revealed that some of the escape variants that arose early after infection could still bind to the selecting antibody. In contrast, no antibody binding could be detected for late isolates, indicating that binding, but nonneutralizing, antibodies exerted a selective pressure on the virus. Infection of naive TgH(KL25) mice with distinct escape viruses differing in their antibody-binding properties revealed that nonneutralizing antibodies accelerated clearance of antibody-binding virus variants in a partly complement-dependent manner. Virus variants that did not bind antibodies were not affected. We therefore conclude that nonneutralizing antibodies binding to the same antigenic site as neutralizing antibodies are biologically relevant by limiting early viral spread.

Toll-like receptor engagement converts T-cell autoreactivity into overt autoimmune disease.

Autoimmune diabetes mellitus in humans is characterized by immunological destruction of pancreatic beta islet cells. We investigated the circumstances under which CD8(+) T cells specific for pancreatic beta-islet antigens induce disease in mice expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) as a transgene under the control of the rat insulin promoter. In contrast to infection with LCMV, immunization with LCMV-GP derived peptide did not induce autoimmune diabetes despite large numbers of autoreactive cytotoxic T cells. Only subsequent treatment with Toll-like receptor ligands elicited overt autoimmune disease. This difference was critically regulated by the peripheral target organ itself, which upregulated class I major histocompatibility complex (MHC) in response to systemic Toll-like receptor-triggered interferon-alpha production. These data identify the 'inflammatory status' of the target organ as a separate and limiting factor determining the development of autoimmune disease.

Innate immune-induced depletion of bone marrow neutrophils aggravates systemic bacterial infections.

Neutrophils are the most abundant leukocytes in circulation and provide a primary innate immune defense function against bacterial pathogens before development of a specific immune response. These specialized phagocytes are short lived (12-24 hours) and continuously replenished from bone marrow. We found that if the host is overwhelmed by a high inoculum of Listeria monocytogenes, neutrophils are depleted despite high granulocyte-colony stimulating factor induction. In contrast to a low-dose innocuous L. monocytogenes infection, high-dose Listeria challenge blocks neutrophil recruitment to infectious abscesses and bacterial proliferation is not controlled, resulting in lethal outcomes. Administering synthetic TLR2-ligand or heat-killed bacteria during the innocuous L. monocytogenes infection reproduced these effects, once again leading to overwhelming bacterial propagation. The same stimuli also severely aggravated Salmonella typhimurium, Staphylococcus aureus, and Streptococcus pyogenes systemic infection. These data implicate systemic innate immune stimulation as a mechanism of bone marrow neutrophil exhaustion which negatively influences the outcome of bacterial infections.

Long-lasting immunity by early infection of maternal-antibody-protected infants.

Newborn higher vertebrates are largely immuno-incompetent and generally survive infections--including poxviruses--by maternal antibody protection. Here, we show that mice survived epidemics as adults only if exposed to lethal orthopoxvirus infections during infancy under the umbrella of maternal protective antibodies. This implies that both the absence of exposure to infection during early infancy or of effective vaccination renders the population highly susceptible to new or old re-emerging pathogens.

Tissue macrophages suppress viral replication and prevent severe immunopathology in an interferon-I-dependent manner in mice.

UNLABELLED: The innate immune response plays an essential role in the prevention of early viral dissemination. We used the lymphocytic choriomeningitis virus model system to analyze the role of tissue macrophages/Kupffer cells in this process. Our findings demonstrated that Kupffer cells are essential for the efficient capture of infectious virus and for preventing viral replication. The latter process involved activation of Kupffer cells by interferon (IFN)-I and prevented viral spread to neighboring hepatocytes. In the absence of Kupffer cells, hepatocytes were not able to suppress virus replication, even in the presence of IFN-I, leading to prolonged viral replication and severe T cell-dependent immunopathology. CONCLUSION: Tissue-resident macrophages play a crucial role in early viral capture and represent the major liver cell type exhibiting responsiveness to IFN-I and providing control of viral replication.

Antagonistic variant virus prevents wild-type virus-induced lethal immunopathology.

Altered peptide ligands (APLs) and their antagonistic or partial agonistic character-influencing T cell activation have mainly been studied in vitro Some studies have shown APLs as a viral escape mechanism from cytotoxic CD8(+) T cell responses in vivo. However, whether infection or superinfection with a virus displaying an antagonistic T cell epitope can alter virus-host relationships via inhibiting T cell-mediated immunopathology is unclear. Here, we evaluated a recently described CD4(+) T cell escape lymphocytic choriomeningitis virus (LCMV) variant that in vitro displayed antagonistic characteristics for the major histocompatibility complex class II-restricted mutated epitope. Mice transgenic for the immunodominant LCMV-specific T helper epitope that usually succumb to wild-type LCMV-induced immunopathology, survived if they were simultaneously coinfected with antagonistic variant but not with control virus. The results illustrate that a coinfecting APL-expressing virus can shift an immunopathological virus-host relationships in favor of host survival. This may play a role in poorly cytopathic long-lasting virus carrier states in humans.

Microchimerism maintains deletion of the donor cell-specific CD8+ T cell repertoire.

Rare cases of stable allograft acceptance after discontinuation of immunosuppression are often accompanied by macrochimerism (> 1% donor cells in blood) or microchimerism (< 1% donor cells in blood). Here, we have investigated whether persistence of donor cells is the cause or the consequence of long-lasting CTL unresponsiveness. We found that engraftment of splenocytes bearing a single foreign MHC class I-restricted epitope resulted in lifelong donor cell microchimerism and specific CTL unresponsiveness. This status was reversed in a strictly time- and thymus-dependent fashion when the engrafted cells were experimentally removed. The results presented herein show that microchimerism actively maintains CTL unresponsiveness toward a minor histocompatibility antigen by deleting the specific repertoire and thus excluding dominant, T cell extrinsic mechanisms of CTL unresponsiveness independent of systemically persisting donor cell antigen.

"Viral déjà vu" elicits organ-specific immune disease independent of reactivity to self.

Autoimmune diseases are often precipitated by viral infections. Yet our current understanding fails to explain how viruses trigger organ-specific autoimmunity despite thymic tolerance extending to many non-lymphohematopoietic self antigens. Additionally, a key epidemiological finding needs to be explained: In genetically susceptible individuals, early childhood infections seem to predispose them to multiple sclerosis (MS) or type 1 diabetes years or even decades before clinical onset. In the present work, we show that the innate immune system of neonatal mice was sufficient to eliminate an attenuated lymphocytic choriomeningitis virus (LCMV) from most tissues except for the CNS, where the virus persisted in neurons (predisposing virus). Virus-specific cytotoxic T cells (CTLs) were neither deleted nor sufficiently primed to cause disease, but they were efficiently triggered in adulthood upon WT LCMV infection (precipitating virus). This defined sequence of viral infections caused severe CNS inflammation that was histomorphologically reminiscent of rasmussen encephalitis, a fatal human autoimmune disease. Yet disease in mice was mediated by antiviral CTLs targeting an epitope shared by the precipitating virus and the predisposing virus persisting in neurons (déjà vu). Thus the concept of "viral déjà vu" demonstrates how 2 related but independently encountered viral infections can cause organ-specific immune disease without molecular mimicry of self and without breaking self tolerance.

Immunoprivileged status of the liver is controlled by Toll-like receptor 3 signaling.

The liver is known to be a classical immunoprivileged site with a relatively high resistance against immune responses. Here we demonstrate that highly activated liver-specific effector CD8+ T cells alone were not sufficient to trigger immune destruction of the liver in mice. Only additional innate immune signals orchestrated by TLR3 provoked liver damage. While TLR3 activation did not directly alter liver-specific CD8+ T cell function, it induced IFN-alpha and TNF-alpha release. These cytokines generated expression of the chemokine CXCL9 in the liver, thereby enhancing CD8+ T cell infiltration and liver disease in mice. Thus, nonspecific activation of innate immunity can drastically enhance susceptibility to immune destruction of a solid organ.

Kinetics of protective antibodies are determined by the viral surface antigen.

Delayed and weak virus neutralizing antibody (nAb) responses represent a hallmark correlating not only with the establishment of persistent infection but also with unsuccessful vaccine development. Using a reverse genetic approach, we evaluated possible underlying mechanisms in 2 widely studied viral infection models. Swapping the glycoproteins (GPs) of lymphocytic choriomeningitis virus (LCMV, naturally persisting, noncytolytic, inefficient nAb inducer) and vesicular stomatitis virus (VSV, nonpersisting, cytolytic, potent nAb inducer) transferred the only target of nAb's from either virus to the other. We analyzed the nAb response to each of the 2 recombinant and parent viruses in infected mice and found that nAb kinetics were solely determined by the viral surface GP and not by the virus backbone. Moreover, the slowly and poorly nAb-triggering LCMV virion was a potent immunogenic matrix for the more antigenic VSV-GP. These findings indicate that the viral GP determines nAb kinetics largely independently of the specific viral infection context. They further suggest that structural features of viral GPs or coevolutionary adaptation of the virus's GP to the host's naive B cell repertoire, or both, may critically limit nAb kinetics and improvement of vaccine efficacy.

On differences between immunity and immunological memory.

The evolutionary benefits of immunological memory are important: whereas antibodies can be transmitted to offspring by their mother and thereby benefit the species, T cell memory may function to help the individual combat persistent infection in peripheral tissues. Although experimental immunological memory is largely maintained antigen-independently, protective immunity is antigen-dependent.

Immunological memory: lessons from the past and a look to the future.

Immunological memory is considered to be one of the cardinal features of the adaptive immune system. Despite being a recognized phenomenon since the time of the ancient Greeks, immunologists are yet to fully appreciate the mechanisms that control memory responses in the immune system. Furthermore, our definition of immunological memory itself continues to evolve, with recent suggestions that innate immune cells also show memory-like behaviour. In this Viewpoint article, Nature Reviews Immunology invites five leading immunologists to share their thoughts on our current understanding of the nature of immunological memory. Our experts highlight some of the seminal studies that have shaped the immune memory field and offer contrasting views on the key questions that remain to be addressed.

Arterial inflammation and atherosclerosis.

Arterial inflammation is a significant component of atherosclerotic disease and it has been suggested that specific immune responses directed against autoantigens or pathogen-derived antigens presented in the vascular wall could initiate and/or maintain atherosclerotic processes. Atherogenic cofactors such as altered cholesterol metabolism may not only impact locally on inflammatory responses in atherosclerotic lesions, but may also alter general immune-responsiveness. The evidence to date suggests that the mutual chronic perpetuation of immune mediated vascular inflammation and cholesterol-induced atherosclerosis is a key step in atherogenesis.

Antiinfection immunity and autoimmunity.

This review summarizes findings that indicate that immune reactions of T and B cells depend on thresholds of the binding avidity of the receptor, on antigen amounts, on the time period during which antigen is available in secondary lymphatic tissues, as well as on the detection methods used. Usually immunologically ignored, strictly extralymphatic host cells or their antigens may be released by infection or immunopathology to reach lymphatic organs and induce autoimmune diseases. If we know the infection, we call the disease immunopathologically mediated; if we do not recognize or know it, we call the disease autoimmune.